cgroup/cpuset: Code cleanup and comment update

Rename partition_xcpus_newstate() to isolated_cpus_update(),
update_partition_exclusive() to update_partition_exclusive_flag() and
the new_xcpus_state variable to isolcpus_updated to make their meanings
more explicit. Also add some comments to further clarify the code.
No functional change is expected.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>